Hood on Hackage is an interesting way to see intermediate data structures. {-# LANGUAGE NoMonomorphismRestriction #-} -- Fold Behaviour Observed module Folding where -- See Hood on Hackage import Observe import Data.List n = 10::Int fr = foldr (observe "Add" (+)) 0 [1..n] fl = foldl (observe "Add" (+)) 0 [1..n] frr = foldr (observe "Add" (+)) 0 (reverse [1..n]) flr = foldl (observe "Add" (+)) 0 (reverse [1..n]) fro = printO fr flo = printO fl frro = printO frr flro = printO flr fl' = foldl' (observe "Add" (+)) 0 [1..n] flr' = foldl' (observe "Add" (+)) 0 (reverse [1..n]) flo' = printO fl' flro' = printO flr'